An important area of speech recognition is automatic recognition of connected digit strings (i.e., strings composed of the digits 0 through 9—including “oh”). Applications of this technology include credit card authorization, catalog ordering, dialing of telephone numbers, and data entry, to name just a few. For the past 2 yr AT&T, in cooperation with American Express Travel Related Services, has experimented with a system for automatic recognition of 10‐digit merchant identification codes, and 15‐digit customer credit card numbers, for the purpose of authorizing purchases charged to an AMEX card. (The problem of recognizing the transaction amount has also been studied, but this is a much more difficult problem and will not be discussed.) Field trial experience with the recognizer using about 1000 customers who provided 2000 connected digit strings over 800‐based dialed up telephone connections was correct recognition of 97%–98% of the strings with no rejections using constraints on the validity of both merchant identifications and credit card numbers. Several schemes for applying the constraints in a practical implementation were studied and will be discussed in the talk.